I have never downshifted. When you're slowing down or coming to a stop, there is no sense in continuing to have your RPM's up which is an accurate indicator of how much gas you are using. The more you can have your car going at the necessary speed with the lowest RPM's possible, the better fuel economy you will get. (Within reason, don't strain your transmission trying to drive 10 MPH in 4th gear for low RPM's.) If I am going down a hill, coming to a stop sign/stoplight, or am coming to a speed limit reduction, I put the car in neutral and let it coast.
With only regular oil changes and an occasional bottle of fuel injector cleaner I have been receiving an average of 31-33 MPG which is a mix of town and highway driving. (average suburban driving).